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Muscle spindles are fusiform (spindle-shaped) and are found within the belly of a skeletal muscle. They are composed of 5-14 muscle fibres, with each spindle containing multiple intrafusal muscle fibres. These intrafusal fibres are innervated by efferent gamma motor neurons, which modify the sensitivity of the muscle spindle sensory afferents to stretch. The muscle spindles and golgi tendon organs are proprioceptive sensory organs, which detect changes in muscle length, posture, and movement of body parts. They inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching, allowing the CNS to compute the position and movement of our extremities in space.
| Characteristics | Values |
|---|---|
| Location | Within the belly of a skeletal muscle |
| Shape | Spindle-shaped |
| Composition | 5-14 muscle fibres |
| Types of Muscle Fibres | Dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres |
| Motor Neurons | Gamma motor neurons (also known as fusimotor neurons) and beta motor neurons |
| Function | Inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching |
| Sensory Receptors | Delicate sensory receptors or proprioceptors |
| Response to Changes in Length | Activating motor neurons via the stretch reflex to resist muscle stretch |
| Role in Diseases | Many neuromuscular diseases affect muscle spindle function, contributing to an unstable gait, frequent falls, and ataxic behaviour |
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What You'll Learn
Muscle spindles contain intrafusal muscle fibres
Muscle spindles are fusiform structures that are 0.5–4 mm in length and found within the belly of a skeletal muscle. They are surrounded by a thick capsule of connective tissue and contain multiple small intrafusal muscle fibres, nerve fibres, specialised nerve endings, and blood vessels. The muscle fibres inside the spindle are called intrafusal, while those outside are called extrafusal.
Intrafusal muscle fibres are a type of specialised muscle fibre found within muscle spindles. They are responsible for sensing changes in muscle length, posture, and movement of body parts. The central region of an intrafusal muscle fibre does not contract, while the ends do. This is because contractile proteins such as actin and myosin are only present at each end of the fibre. The central portion of the intrafusal muscle fibre contains the nuclei of the muscle fibre, and the arrangement of these nuclei determines whether the intrafusal muscle fibres are considered nuclear bag fibres or nuclear chain fibres.
There are three types of intrafusal muscle fibres: dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres. The nuclear bags come in two varieties: static bags and dynamic bags. The static nuclear bag and nuclear chain fibres receive a second type of innervation, classified as II afferents. These innervate the juxtaequatorial regions of the intrafusal fibres. The II afferents consist of medium myelinated fibres that adapt slowly, carrying information about static muscle length.
The intrafusal muscle fibres are innervated by different neurons. The central (equatorial) part is in contact with afferent proprioceptive sensory neurons, termed primary "group Ia afferents" and secondary "group II afferents". In addition, intrafusal muscle fibres are innervated by efferent gamma motoneurons in both polar regions, where they form a cholinergic synapse. The polar regions of intrafusal fibres contain most of the contractile elements.
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They are made up of dynamic nuclear bag fibres
Muscle spindles are fusiform (spindle-shaped) and are made up of specialised intrafusal muscle fibres. These intrafusal muscle fibres are of three types: dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres.
Dynamic nuclear bag fibres are a type of intrafusal muscle fibre that lies in the centre of a muscle spindle. Each has many nuclei concentrated in bags and they cause excitation of the primary sensory fibres. There are two kinds of bag fibres based upon contraction speed and motor innervation. BAG2 fibres are the largest. They have no striations in the middle region and swell to enclose nuclei, hence their name. BAG1 fibres are smaller than BAG2 fibres. Both bag types extend beyond the spindle capsule. These sense the dynamic length of the muscle. They are sensitive to length and velocity.
The dynamic response of de-efferented spindle afferents comes from sensory endings on the bag2 fibre, and static responses come from sensory endings on chain fibres. Dynamic gamma activity is adjusted during movement preparation to facilitate the execution of the planned action. For example, if the intended movement direction is associated with the stretch of the spindle-bearing muscle, Ia afferent and stretch reflex sensitivity from this muscle is reduced.
Gamma fusimotor control allows for the independent preparatory tuning of muscle stiffness according to task goals. The function of the gamma motor neurons is to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibres contract, thus elongating the non-contractile central portions. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents.
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They contain gamma motor neurons
Muscle spindles are innervated by sensory neurons and motor neurons to provide proprioception and facilitate appropriate movements via the firing of motor neurons. They contain gamma motor neurons, which are a type of lower motor neuron that innervates intrafusal muscle fibres within the muscle spindle. Gamma motor neurons are smaller than alpha motor neurons and are responsible for maintaining the sensitivity of muscle spindles to changes in muscle length.
The central nervous system (CNS) controls muscle spindle sensitivity through the fusimotor system, which consists of muscle spindles and gamma motor neurons, also known as fusimotor neurons. Gamma motor neurons are the efferent part of the fusimotor system, sending signals away from the CNS. They play a crucial role in alpha-gamma coactivation, a process that involves the simultaneous firing of both alpha and gamma motor neurons to maintain the tautness of muscle spindles.
During alpha-gamma coactivation, the activation of gamma motor neurons causes a weak contraction of the intrafusal fibres, keeping the spindle taut and sensitive to changes in muscle length. This coordination between alpha and gamma motor neurons ensures that muscle spindles can detect and respond to stretch over a wide range of muscle lengths. The intrafusal fibres innervated by gamma motor neurons are located within the muscle spindle and are oriented parallel to the extrafusal muscle fibres.
Gamma motor neurons are essential for maintaining muscle spindle function and overall motor control. They help regulate muscle tone and contribute to the CNS's ability to compute the position and movement of our limbs in space. Additionally, gamma motor neurons play a role in preventing temporary insensitivity of muscle spindles by maintaining their tautness and sensitivity during muscle contractions. Without gamma motor neurons, muscle spindles would become very loose during muscle contractions, leading to imprecise stretch detection.
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They contain beta motor neurons
Muscle spindles are encapsulated organs that range from 4 to 10 mm in length. They are collections of 6–8 specialised muscle fibres that are located within the muscle mass itself. These fibres are known as intrafusal fibres and are found within extrafusal (skeletal) muscle fibres. Each muscle contains many muscle spindles, and they are important for maintaining posture and stable gait.
The motor part of the spindle is provided by motor neurons, including gamma motor neurons (also known as fusimotor neurons) and beta motor neurons. Gamma motor neurons activate the muscle fibres within the spindle, causing a contraction and stiffening of the end parts of the muscle spindle muscle fibres. Beta motor neurons, on the other hand, supply muscle fibres both within and outside of the spindle.
Beta motor neurons play a crucial role in muscle function by supplying muscle fibres within and outside the spindle. When activated, they cause a contraction of the muscle fibres, contributing to the overall muscle movement and coordination. The activation of beta motor neurons is particularly important when a burst of large amounts of force is required, such as during an escape mechanism.
The coordination of muscle spindles and beta motor neurons is essential for maintaining muscle health and function. Any disruption to this coordination, such as an upper motor neuron lesion, can lead to abnormalities in muscle tone and function. Therefore, the presence and proper functioning of beta motor neurons within muscle spindles are vital for ensuring the overall well-being and performance of the muscles.
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They are found within the belly of a skeletal muscle
Muscle spindles are found within the belly of a skeletal muscle. They are fusiform (spindle-shaped), and the specialised fibres that make up the muscle spindle are called intrafusal muscle fibres. Each muscle spindle contains multiple intrafusal muscle fibres, which have contractile proteins like actin and myosin. However, these contractile proteins are only present at each end of an intrafusal muscle fibre, meaning that the central region does not contract. The central portion of the intrafusal muscle fibre contains the muscle fibre's nuclei, and the arrangement of the nuclei determines whether the intrafusal muscle fibres are considered nuclear bag fibres or nuclear chain fibres.
The muscle spindles and Golgi tendon organs are proprioceptive sensory organs, which detect changes in muscle length, posture, and the motion of body parts. They are delicate sensory receptors that inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, maintaining posture, and a stable gait.
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons (also known as fusimotor neurons) and, to a lesser extent, by one or two beta motor neurons. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle. Activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
The function of the gamma motor neurons is not to supplement the force of muscle contraction provided by the extrafusal fibres, but to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibres contract, thus elongating the non-contractile central portions. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents.
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Frequently asked questions
Muscle spindles are delicate sensory receptors that inform the central nervous system about changes in the length of individual muscles and the speed of stretching.
Each muscle spindle contains multiple intrafusal muscle fibres. These fibres have contractile proteins like actin and myosin, but these are only present at each end of the fibre. The central region of the intrafusal muscle fibre contains the muscle fibre's nuclei.
Intrafusal muscle fibres are found within the belly of a skeletal muscle. They are the specialised fibres that make up the muscle spindle and are much thinner than extrafusal muscle fibres.
There are three types of intrafusal muscle fibres: dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres.
Muscle spindles trigger the stretch reflex, which prevents muscles from being overstretched. They also play a critical role in sensorimotor development and body awareness.
